1. Field of the Invention
This invention relates to spark discharge, electro-erosion machine tools. More particularly, this invention relates to a control system and fluid supply apparatus for regulating the pressure and flow of the flushing fluid that flows through the working gap between a wire and a workpiece in the operation of an electrical discharge machining (EDM) apparatus and for providing flushing at higher pressures for improved performance.
2. Description of the Prior Art
Spark discharge, electro-erosion or electrical discharge machining (EDM) apparatus make use of controlled electrical discharges or sparks to cut through or erode electrically-conductive workpieces. The cutting is achieved by high frequency electrical discharges between a wire that acts as an electrode and a workpiece that acts as a counter-electrode. Pulses of suitable electrical voltage are applied across a gap between the wire and the workpiece known as the working gap, or working area. A dielectric liquid, normally de-ionized water, or oil, is supplied to the working gap. When the voltage across the working gap becomes sufficiently high the dielectric liquid becomes ionized at a point along the wire, breaking down electrically to form an instantaneous and intense conductive channel between the wire and the workpiece. The concentration of high energy at a localized site causes the removal of a discrete quantity of metal from the workpiece. By moving the workpiece relative to the wire, intricate and precise cuts can be made in the workpiece. A numerical control system is generally used to regulate the movement of the workpiece to achieve the desired profile. An example of a process control strategy of an EDM machine and associated numerical control system can be found in Buhler U.S. Pat. No. 4,533,811.
It is important in many applications for the EDM machine to provide cuts of extremely high precision and accuracy. The dielectric medium is circulated through the working zone by the action of flushing. As the cutting operation proceeds, the amount of ions, or ionization, in the dielectric fluid increases. In other words, the deionized water or flushing fluid increases its conductivity when it is contaminated by the products of the erosion process. The fluid must therefore be periodically filtered and deionized in order to maintain the proper conductivity level of the fluid, or the increased conductivity can cause irregularities in the cutting width and exceed the required, precise tolerances.
By continual flushing flow, the contaminated and ionized liquid resulting from the machining action is flushed from the work area providing a replenished source of de-ionized water for subsequent machining. The flushing action also cools the current-carrying wire, thereby reducing the chance of breakage due to current induced heat.
In prior art flushing systems, flushing fluid flow has been provided by a single multi-stage centrifugal pump that supplies a flow that is divided to provided fluid to both upper and lower flushing nozzles. The automatic control of such pumps in prior art devices has been somewhat arbitrary due to the lack of a specific relationship between the pump operating speed and the pressure or flow output. In addition, differences in flow required through upper and lower nozzles must be achieved through manual control of valves positioned in the supply lines feeding the upper and lower nozzles by the machine operator.